Piezoelectric crystal element



Oct. 4, 1949. JAFFE 2,483,640

PIEZOELECTRIC CRYSTAL ELEMENT Filed Feb. 16, 1948 INVENTOR. HANS JAFFE RNEY Patented Oct. 4, 1949 2,483,840 PIEZOELECTBIC CRYSTAL ELEMENT Hans Jaile, Cleveland, Ohio, asaignor to The Brush Developmenhtifompany, Cleveland, Ohio,

a corporation of 0 Application February 16, 1948, Serial No. 8,677

6 Claims. (Cl. 171-327) This invention pertains to a piezoelectric crystal element cut with particular orientation from any one of a number of representatives or a family of crystals.

This application is a continuation-in-part of a copending application filed June 8, 1944 in the name of Hans Jafle, Serial Number 539,312, (now Patent No. 2,463,109, issued March 1, 1949) for Piezoelectric crystal means.

The present application is based on the unexpected discovery that a plate-like piezoelectric crystal element cut from an arsenate crystal of the P-type with its thickness direction extending in a direction substantially perpendicular to the Z-axis of the crystal has outstanding characteristics.

The term P-type crystal" is to be understood as comprising primary ammonium phosphate (NH4H2PO4), primary potassium phosphate, primary rubidium phosphate, the primary arsenates of ammonium potassium and rubidium, isomorphous mixtures of any of these named compounds, and all other piezoelectrically active crystalline materials isomorphous therewith. All of these crystals belong to the crystallographic symmetry class Va, also known as the di-tetragonal alternating crystal class, and as the tetragonal sphenoidal class. It is characterized by the presence of three two-fold axes of symmetry perpendicular to each other and two planes of symmetry at right angles to each other and intersecting in one of the twofold axes. The planes cut the other two twofold axes at angles of 45 degrees. This combination of symmetry elements makes that axis which is parallel to the two planes of symmetry 3, fourfold alternating symmetry axis which is also the optic axis of the crystal.

Of the several P-type crystals, primary ammonium phosphate (NH4H2PO4) has been found to be of particular merit, and a certain cut thereof is claimed in the copending application Serial Number 539,312 (now Patent No. 2,463,109). As explained in thatapplication, the useful piezo electric properties of the primary ammonium phosphate crystal are due to the high value of its piezoelectric coeflicient (136 which expresses the interaction of an electric field along the Z-axis of the crystal with mechanical strains in the plane perpendicular to the Z-axis. The other piezoelectric coeflicient present in crystals of the P-type is the coeflicient 1114 describing the action of an electric field component in a plane perpendicular to the Z-axis. It was stated in the aforesaid Patent No. 2,463,109 that in primary ammonium phosphate this coeflicient dis is very 2 small. The same is also true about the well known crystal primary potassium phosphate. 1-

have made the unexpected and significant discovery that the corresponding arsenate crystals, 1. e., primary ammonium arsenate and primary potassium arsenate, in spite of the chemical and crystallographic similarity to the corresponding coulomb/newton. The same coefilcient for primary ammonium phosphate and primary potassium phosphate, respectively, is 1.5 and 1.3x10- coulomb/ newton.

It is of particular interest to note that in both primary ammonium arsenate and primary potassium arsenate the sign of the coefficients du and die is the same, so that the action of these two coefflcients adds up in those cuts which depend on the combined action of the two coefflcients, while in primary ammonium phosphates the coefllcients dac and d1;I are of opposite sign. A family of cuts depending onthe combined action of the coefllcients (114 and dae of P-type crystals is shown in my copending application Serial Number 8,678, filed concurrently herewith.

For the family of cuts herein claimed, primary ammonium arsenate and primary potassium arsenate are of particularmerit. Both of these crystals are stable up to at least 120 degrees centigrade. They contain no water of crystallization so that they may be subjected to a vacuum for long periods of time without detrimental effects. A further advantage of using an element cut from a crystal of the P-type is that when it is used in an enclosed transducer housing, an amount of an extreme drying agent such as phosphorous pentoxide may be added to prevent moisture from establishing shunt circuits across the element faces between the electrodes. Previously, when silica gel or the like was put into a transducer housing containing a Rochelle salt element, care had to be exercised to see that the silica gel was only partially dry, as completely dehydrated silica gel would rob the Rochelle salt of its water of crystallization, thereby rendering the unit inoperative.

For a better understanding of the present in- 55 vention, together with other and further objects perpendicular 3 thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawing the figure is a schematic isometric view of a P-type crystal showing several of the family of preferred plates which may be cut therefrom.

Each of the three crystal plates ill, ll, 12

' shown in the figure has its thickness direction perpendicular to the Z-axis, and it is to be understood that in operation the electric field direction is to be in the thickness direction of the plates. The crystal plate indicated by the reference character I is an X-zero cut plate which will be excited to face shear vibrations by an alternating field applied in its thickness direction. The plate identified by reference character H is an X-45 degree or expander plate which will expand and contract in its length direction upon application of an alternating field in its thickness direction. The crystal plate i2 has its thickness direction at 45 degrees to the X-axis and is called a thickness shear plate as it is excited to shear vibrations around its length direction by an electric field applied in the thickness direction. Other cuts with their thickness direction perpendicular to the Z-axis may be made and will, in general, show vibrations which are a combination of the simple vibrations produced in the described cuts III, II and I2. These other cuts are indicated by the construction line i3.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim as my invention:

1. A plate-like piezoelectric crystal element cut from an arsenate crystal of the P-type, the

thickness direction of said plate extending in a direction substantially perpendicular to the Z- axis of said crystal.

2. A plate-like piezoelectric crystal element cut from an arsenate crystal of the P-type, the thickness direction of said plate extending in a direction substantially parallel to a line bisecting the angle between the X and Y axes of the crystalline material.-

3. A plate-like piezoelectric crystal element cut from a primary ammonium arsenate crystal of the P-typ the thickness direction of said plate extending in a direction substantially perpendicular to the Z-axis oi? said crystal.

4. A plate-like piezoelectric crystal element cut from a primary ammonium arsenate crystal of the P-type, the thickness direction of said plate extending in a direction substantially parallel to a line bisecting the angle between the X and Y axes of the crystalline material.

5. A plate-like piezoelectric crystal element cut from a primary potassium arsenate crystal of the P-type, the thickness direction of said plate extending in a direction substantially perpendicular to the Z-axis of said crystal.

6. A plate-like piezoelectric crystal element cut from a primary potassium arsenate crystal of the P-type, the thickness direction of said plate extending in a direction substantially parallel to a line bisecting the angle between the X and Y axes of the crystalline material.

HANS JAFFE.

REFERENCES CITED UNITED STATES PATENTS Name Date Baewald April-10, 1945 OTHER REFERENCES Mason, Physical Review, volume 69, page 181, March 1946. (Copy in Scientific Library.)

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